An internal combustion engine may be supplied fuel from a fuel tank. The fuel tank may contain fuel vapors that increase pressure in the fuel tank above atmospheric pressure. Pressure in the fuel tank may be a motive force for driving fuel vapors from the fuel tank to a fuel vapor storage canister. One side of the fuel vapor storage canister may be vented to atmosphere so that fuel vapors flow into the fuel vapor storage canister. Once fuel vapors enter the fuel vapor storage canister they may be adsorbed in activated carbon. The activated carbon strips fuel from the gaseous mixture entering the fuel vapor storage canister and allows air to be released to atmosphere, thereby reducing pressure in the fuel tank and the fuel vapor storage canister. A canister purge valve located along a line or conduit leading from the fuel vapor canister to the engine intake manifold may be opened from time to time to purge fuel vapors from the fuel vapor storage canister. A lower pressure in the engine intake manifold may draw air from atmosphere through the fuel vapor canister where it combines with the stored fuel vapors before entering the engine. The concentration of fuel within the air-fuel mixture drawn from the fuel vapor storage canister may range from mostly air to mostly fuel. If the concentration of fuel in vapors drawn to the engine is high, engine cylinders may be exposed to fuel amounts that may be sufficient to cause misfire within the engine cylinders.
The internal combustion engine may also enter a mode referred to as deceleration fuel shut off (DFSO) or deceleration fuel cut out during lower driver demand torque conditions. During DFSO, fuel supplied to engine cylinders ceases while the engine continues to rotate via the vehicle's kinetic energy that is delivered to the engine through the vehicle's wheels and powertrain. By ceasing fuel delivery to the engine during periods of low driver demand, engine fuel consumption may be reduced. However, if fuel vapors are drawn into engine cylinders from the fuel vapor canister when the canister purge valve is closed due to low engine intake manifold pressure, engine cylinders may misfire when the engine cylinders are reactivated. Therefore, it may be desirable to provide a way of reducing the possibility of engine misfire when engine cylinders are reactivated after an engine exits DFSO.
The inventor herein has recognized that engine misfire during cylinder reactivation is undesirable and has developed an engine operating method, comprising: holding intake and exhaust valves of a cylinder closed over one or more engine cycles via a controller in response to deceleration fuel shut off conditions being present and an amount of fuel vapor stored in a canister being greater than a threshold.
By closing and holding closed intake and exhaust valves of a cylinder when deceleration fuel shut off conditions are present, it may be possible to provide the technical result of reducing the possibility of cylinder misfire when cylinders resume combusting air and fuel after exiting DFSO. In one example, air may be trapped in engine cylinders as the engine enters DFSO so that the cylinders may be restarted without excess fueling due to fuel vapors entering cylinders during DFSO. In this way, at least one cylinder of an engine may be reactivated after exiting DFSO with a higher degree of confidence.
The present description may provide several advantages. In particular, the approach may provide improved engine reactivation after the engine enters DFSO. Further, the approach may reduce engine air flow to a catalyst in the engine's exhaust system when the engine is in DFSO so that less fuel may be injected to engine cylinders to balance catalyst oxidants and reductants. Additionally, the approach may lower intake manifold vacuum during DFSO so that the possibility of drawing fuel vapors into the engine from the fuel vapor storage canister during DFSO may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.